Mammals evolved much faster after dinosaur extinction than previously thought

Researchers have discovered three early mammal species, one of which has been named for a character from JRR Tolkien's The Hobbit.

When an asteroid struck the Yucatán Peninsula in Mexico 66 million years ago, the abrupt change in the climate led to a mass extinction. The subsequent dinosaur-shaped hole in the ecosystem was filled by a group of species that we know well: the mammals.

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The ‘Age of Mammals’ (also known as the Cenozoic) is the name of the Earth’s current geological era, so called because mammals are the planet’s dominant terrestrial species. It began after the demise of the dinosaurs – but new research suggests that early mammals diversified much more rapidly than previously thought.

Paleontologists from the University of Colorado in Boulder have discovered three new species of mammal that lived only a few hundred thousand years after the dinosaur extinction. Each of these has dental features that are different to other species alive at the time.

“When the dinosaurs went extinct, access to different foods and environments enabled mammals to flourish and diversify rapidly in their tooth anatomy and evolve larger body size,” said lead author Madelaine Atteberry from the University of Colorado Geological Sciences Department.

“They clearly took advantage of this opportunity, as we can see from the radiation of new mammal species that took place in a relatively short amount of time following the mass extinction.”

Left to right, Conacodon hettingeri, Miniconus jeanninae, Beornus honeyi © Banana Art Studio
Left to right, Conacodon hettingeri, Miniconus jeanninae, Beornus honeyi © Banana Art Studio

The three species are ‘archaic ungulates’, the ancestors of modern hoofed animals. They were discovered through fossils of teeth and jaw bones unearthed from the Great Divide Basin in Wyoming, USA.

Though the species could have been herbivores, the researchers believe they may have been omnivores: their teeth would have allowed them to eat both meat and plants.

The largest of the three is named after The Hobbit character Beorn, a shape-changer who can take the form of a man or a bear. Beornus honeyi has large premolars and molars, suggesting it was around the size of a cat – much larger than the mouse- or rat-sized mammals that lived in the time of dinosaurs.

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The other two species are named Conacodon hettingeri and Miniconus jeanninae. They are both similar to other species that lived around the same time, but distinct features of their teeth set them apart.

“Previous studies suggest that in the first few hundred thousand years after the dinosaur extinction (what is known in North America as the early Puercan) there was relatively low mammal species diversity across the Western Interior of North America, but the discovery of three new species in the Great Divide Basin suggests rapid diversification following the extinction,” said Atteberry.

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“We haven’t yet fully captured the extent of mammalian diversity in the earliest Paleocene, and predict that several more new species will be described.”

Reader Q&A: What did mammals evolve from?

Asked by: Jamie Horner, Rotherham

Mammals are animals whose females have mammary glands and produce milk, and that includes us. These glands don’t survive in fossils, so most of what we know about mammal evolution depends on the fact that mammals use two small bones for hearing, which other animals, like lizards and dinosaurs, used for eating.

Although there was no abrupt transition to ‘true mammals’, the general idea is that the tetrapods (vertebrates with four legs) divided into amphibians (that lay eggs in water) and amniotes (that lay eggs on land). Amniotes then split into sauropsids (including dinosaurs) and synapsids (including mammal-like reptiles), which eventually led to mammals. Once the dinosaurs were gone, early mammals could stop living nocturnally and flourish in the many forms we find today.

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